1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor, and more particularly to an electrophotographic photoreceptor having at least a charge blocking layer, a moiré preventing layer and a photosensitive layer including an azo pigment. In addition, the present invention also relates to an image forming apparatus and a process cartridge using the photoreceptor.
2. Discussion of the Background
Recently, development of information processing systems utilizing electrophotography is remarkable. In particular, optical printers in which information converted to digital signals is recorded using light have been dramatically improved in print qualities and reliability. This digital recording technique is applied not only to printers but also to copiers, and so-called digital copiers have been developed and used. Copiers utilizing both the conventional analogue recording technique and this digital recording technique have various information processing functions, and therefore it is expected that demand for such copiers will be escalating. In addition, with popularization and improvement of personal computers, the performance of digital color printers which can produce documents including color images has been rapidly improved and broadly used.
Such digital image forming apparatus are required to be improved in functions year by year. Specifically digital image forming apparatus are required not only to have high durability and high stability but also to produce high quality images. On the other hand, in order to produce color images at a high speed, tandem image forming apparatus which include a plurality of image forming units each including image forming devices such as a photoreceptor, a charger, an image irradiator, an image developer, a cleaner and a quencher have been mainly used as the color image forming apparatus. In such tandem image forming apparatus, yellow, magenta, cyan and black image forming units are typically provided side by side, and four color images concurrently formed in the respective image forming units are overlaid on an intermediate transfer medium or a receiving sheet. Thus, a color image can be formed at a high speed. In such tandem image forming apparatus, the image forming devices used therefor are required to be small in size to avoid jumboization of the image forming apparatus. In particular, it is essential that the photoreceptor has a small diameter. However, a photoreceptor which has a smaller diameter but has a shorter life cannot be used. Namely, it is a problem to be solved how to develop a photoreceptor having a small diameter while having a long life.
The life of a photoreceptor mainly depends on two factors, i.e., electrostatic fatigue of the photoreceptor and abrasion of the surface portion thereof. These are problems to be solved of organic photoreceptors, which are mainly used now for electrophotographic image forming apparatus. The former problem (electrostatic fatigue) is that when a photoreceptor is repeatedly subjected to image forming operations such as charging and light irradiating, the electric potentials (potentials of lighted portions and non-lighted portions) formed on the photoreceptor change. In the case of organic photoreceptors, the potential of non-lighted portions typically decreases while the potential of lighted portions (i.e., residual potential) increases after repeated use. The latter problem is that the uppermost layer of a photoreceptor is mechanically abraded after repeated use by members contacting the photoreceptor such as cleaners. If the uppermost layer is thinned due to abrasion, the strength of electric field formed on the photosensitive layer increases, resulting in acceleration of the electrostatic fatigue, and thereby the life of the photoreceptor is further shortened. In addition, when the surface of the photoreceptor is scratched by the contacting members, undesired images (such as streak images) are formed, resulting in shortening of the life of the photoreceptor. Therefore, these problems have to be solved at the same time, to develop a photoreceptor having a long life.
Recently, electrophotographic image forming apparatus can produce images at a high speed. Therefore, the image forming apparatus have also been used for printing fields. In order that electrophotographic image forming apparatus are used for printing fields, color images with high resolution higher than 600 dpi (dots per inch) have to be stably produced. In addition, electrophotographic image forming apparatus have the following advantages over printing machines:                (1) an original image can be directly reproduced at a high speed without making a plate; and        (2) a large number of copies of an original image can be reproduced while a different information image is added to a part of each copy.        
Therefore, the image forming apparatus (systems) are required to have good stability, namely the apparatus is required to stably produce high quality images without producing abnormal images.
As mentioned above, long life and good stability are the important requisites for electrophotographic image forming apparatus. Among the image forming devices included in such electrophotographic image forming apparatus, the photoreceptor is the key device. As a result of studies of the electrostatic properties of photoreceptors and abrasion of the surface of photoreceptors, several technologies have been established.
For example, in order to improve electrostatic properties, charge generation materials having a high photo-carrier generating efficiency have been developed; and charge transport materials having large mobility have been developed. By using a combination of such a charge generation material and a charge transport material, a photoreceptor capable of obtaining large gain and making speedy response in a photo-decaying process can be provided. Therefore, image forming apparatus using such a photoreceptor for an image forming apparatus have the following advantages:                (1) the potential (i.e., non-lighted potential) of the charged photoreceptor can be decreased;        (2) the quantity of light used for optical writing can be decreased;        (3) the developing bias can be decreased;        (4) the transfer bias can be decreased; and        (5) the quenching process can be eliminated.        
Thus, the designing flexibility of the image forming apparatus can be increased. When these factors are minimized, the hazards for the photoreceptor can be eliminated, and thereby the designing flexibility of the photoreceptor can also be increased.
The usage of the photoreceptors used for high speed digital full color image forming apparatus is greatly different from that for analog image forming apparatus and monochrome image forming apparatus. For example, various optical writing methods are used in the full color image forming apparatus. In such full color image forming apparatus, abnormal images are typically produced due to the photoreceptor used. Causes of abnormal images are broadly classified into the following two types. Firstly, abnormal images are caused by scratches formed on the surface of a photoreceptor. Secondly, abnormal images are formed when the photoreceptor used has electrostatic fatigue. In the first case, formation of abnormal images can be prevented to a considerable extent by improving the surface of the photoreceptor (for example, forming a protective layer as an uppermost layer) or improving the contacting members such as cleaners. In the second case, abnormal images (typically, background development) are caused by deterioration of the photoreceptor itself. Among the abnormal images, background development of images produced by a reverse (nega-posi) development method is a big problem now.
Specific examples of the cause for background development are as follows:                (1) soils and defects of the electroconductive substrate used;        (2) dielectric breakdown of the photosensitive layer;        (3) injection of carriers (charges) from the substrate;        (4) increase of dark decay of the photoreceptor; and        (5) carriers thermally generated by the photoreceptor without irradiation of light to the photoreceptor (hereinafter referred to as hot carriers).        
Among these causes, the soils and defects of the electroconductive substrate used can be removed before forming the photosensitive layer thereon, and therefore it is not avoidable. Therefore, in order to prevent occurrence of background development, it is important to improve the electric strength of the photoreceptor so that carrier injection from the substrate is prevented and electrostatic fatigue of the photoreceptor is decreased.
From this point of view, techniques such that an undercoat layer or an intermediate layer is formed between an electroconductive substrate and a photosensitive layer have been proposed. For example, published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 47-6341 discloses an intermediate layer including a nitrocellulose, and JP-A 60-66258 discloses an intermediate layer including a nylon resin. In addition, JP-A 52-10138 discloses an intermediate layer including a maleic acid based resin, and JP-A 58-105155 discloses an intermediate layer including a polyvinyl alcohol resin.
However, these intermediate layers are a resin layer and have a high electric resistance. Therefore, the residual potential of the photoreceptors increases, resulting in decrease of image density when images are formed by a nega-posi developing method. In addition, such intermediate layers exhibit ionic conduction caused by impurities included therein, and therefore the electric resistance thereof increases particularly under low temperature and low humidity conditions, resulting in increase of the residual potential. Therefore, the intermediate layer has to be thinned, and thereby a problem in that the charge properties and charge retainability of the photoreceptor deteriorate after repeated use occurs.
In attempting to solve this problem (i.e., in attempting to control the resistance of an intermediate layer), techniques in that an electroconductive material is included in an intermediate layer have been proposed. For example, JP-A 51-65942 discloses an intermediate layer in which carbon or chalcogen materials is dispersed in a crosslinked resin. JP-A 52-82238 discloses an intermediate layer which is crosslinked using an isocyanate crosslinking agent upon application of heat thereto and which includes a quaternary ammonium salt. JP-A 55-113045 discloses a resinous intermediate layer including a resistance controlling agent. JP-A 58-93062 discloses a resinous intermediate layer including an organic metal compound. However, the photoreceptors including such resinous intermediate layers have a drawback in that images having moiré fringes are produced when the photoreceptors are used for image forming apparatus using coherent light such as laser light for image writing.
In attempting to solve the resistance and moiré fringe problems, intermediate layers including a filler have been proposed. For example, JP-A 58-58556 discloses a resinous intermediate layer including aluminum oxide or tin oxide. JP-A 60-111255 discloses a resinous intermediate layer including a particulate electroconductive material. JP-A 59-17557 discloses an intermediate layer including magnetite. JP-A 60-32054 discloses a resinous intermediate layer including titanium oxide and tin oxide. JP-As 64-68762, 64-68763, 64-73352, 64-73353, 01-118848 and 01-118849 have disclosed resinous intermediate layers including a powder such as borides, nitrides, fluorides and oxides. In these resinous intermediate layers including a filler, the content of the filler in the intermediate layer has to be increased (i.e., the content of the resin has to be decreased) so that the intermediate layer has the desired electric properties. Therefore, the adhesion of the intermediate layer to the electroconductive substrate deteriorates, and thereby a problem in that the intermediate layer is separated from the electroconductive substrate tends to occur. Particularly, when the substrate is a flexible belt, the problem occurs more frequently.
In attempting to solve the problem, formation of a layered intermediate layer has been proposed. The proposed layered intermediate layers are broadly classified into two types, which have structures as illustrated in FIGS. 1 and 2. The first type of the intermediate layers, which is illustrated in FIG. 1, includes an electroconductive substrate 1, and a resin layer 2 including a filler, a resin layer 3 including no filler, and a photosensitive layer 4, which are overlaid in this order. The second type of the intermediate layers, which is illustrated in FIG. 2, includes an electroconductive substrate 1, and a resin layer 3 including no filler, a resin layer 2 including a filler and a photosensitive layer 4 which are overlaid in this order.
Specifically, in the first type intermediate layer, the electroconductive layer 2 includes a filler having a low electric resistance and is formed on the electroconductive substrate 1. In addition, the resin layer 3 is formed thereon. The intermediate layers of this type have been disclosed in JP-As 58-95351, 59-93453, 04-170552, 06-208238, 06-222600, 08-184979, 09-43886, 09-190005 and 09-288367.
In the intermediate layers of this type, the electroconductive layer 2 serves as an electrode. Therefore the intermediate layer is electrically the same as the resinous intermediate layer of the photoreceptor mentioned above, and thereby the above-mentioned electrostatic problem of the photoreceptor having a resinous intermediate layer cannot be solved. Since the electroconductive layer includes a filler, occurrence of moiré fringes can be prevented because the light beam for image writing scatter. When such a photoreceptor is charged, charges having a polarity opposite to that of the charges formed on the surface of the photoreceptor reach the interface between the electroconductive layer 2 and the resinous layer 3. However, when the electroconductive layer 2 has a relatively high resistance, charges are not well injected from the electroconductive substrate 1, and the resistance of the layer 2 increases after long repeated use, thereby increasing the residual potential of the photoreceptor. In addition, in order to avoid the problem caused by defects of the electroconductive substrate 1, the layer 2 has to have a thickness not less than about 10 μm. In this case, the residual potential increasing problem remarkably occurs.
JP-As 05-100461, 05-210260 and 07-271072 have disclosed photoreceptors including an electroconductive layer, and an intermediate layer and a photosensitive layer including a titanylphthalocyanine crystal, which are overlaid in this order. However, the crystal form and the primary particle diameter of the titanyl phthalocyanine crystal are not controlled. Therefore, occurrence of the background development problem due to the hot carriers cannot be prevented.
In the second type intermediate layer, a positive hole blocking layer is formed on the electroconductive substrate, and a resin layer including a filler having a low resistance or an electroconductive filler is formed on the positive hole blocking layer. These intermediate layers have been disclosed in JP-As 05-80572 and 06-19174. The photoreceptors of this type hardly cause the background development problem because the intermediate layer has a positive hole blocking function. In addition, since a filler-including layer is present thereon, residual potential hardly increases. Specifically, injection of positive holes from the electroconductive substrate 1 to the photosensitive layer 4 can be avoided, and thereby the background development problem in a nega-posi development method is hardly caused. In addition, since a charge blocking layer is formed as a lower layer, the degree of increase of residual potential of the photoreceptor after long repeated use is lower than in the case where the charge blocking layer is formed as an upper layer.
However, the background development is caused not only by charges injected from the electroconductive substrate to the photosensitive layer but also by carriers thermally generated in the photosensitive layer. If a proper charge generation material is not used for the charge generation layer and the conditions of the particles of the charge generation material are not properly controlled, occurrence of the background development problem cannot be prevented.
Because of these reasons, a need exists for an electrophotographic photoreceptor which can stably produce high quality images even after long repeated use with hardly causing the background development problem and the dielectric breakdown problem.